The present invention generally relates to analysis of potentially hazardous mixtures of substances and specifically to apparatus and method means for combustibility monitoring by determining the concentration of a first critical component in an environment or atmosphere of interest containing a second component that is capable of forming a combustible mixture with the critical first component. The invention is of particular relevance for monitoring gaseous substances and atmospheres containing them.
Combustible gaseous mixtures are those in which a first gaseous component thereof is capable of rapid exothermic reaction with a second component of the mixture by an oxidative process which may, but need not, involve oxygen as the oxidizing agent. Depending upon the relative amounts of the reacting components, the speed of reactions, the amount of heat generated, and the activation required for initiation of the reaction, such mixtures may cause substantial hazards.
Consequently, prevention of the formation of such mixtures, such as by continuous monitoring of an atmosphere of interest, is of general technological importance, and may be mandatory by law. Such monitoring is of particular importance in substantially enclosed environments, such as housings of processors which generate combustible or explosive gas mixtures by evolving hydrogen, possibly but not necessarily in the presence of a surplus of oxygen, or other relatively enclosed spaces where hazardous or potentially hazardous gas mixtures may occur without human intervention, such as methane in the air of mines, or spaces in areas where gaseous combustible chemicals or fuels are produced, processed, distributed or consumed.
The presence of elemental hydrogen in an atmosphere that contains, or may come into contact with, a sufficient amount of oxygen, such as air, is particularly hazardous because of the extremely high specific amount of heat generated upon reaction between hydrogen and oxygen and the extremely low activation energy required for initiation of the reaction. Thus, hydrogen monitoring is needed in various industrial work-places, e.g. in the immediate vicinities of containers deliberately filled with gaseous hydrogen, such as large electrical transformers and nuclear or chemical reactors as well fuel reservoirs whenever liquid or gaseous hydrogen is used as a fuel. The atmosphere within such containers or a surrounding enclosure will become explosive when the amount of gaseous hydrogen as the first component attains 5%, by volume, if oxygen as the second component is present in an at least stoichiometrically equivalent amount or in stoichiometric excess, or when at least 5%, by volume, of gaseous oxygen as the first component is formed in the presence of a stoichiometric equivalent or excess of gaseous hydrogen as the second component.
However, while monitoring of atmospheres of interest that do contain oxygen and may contain hydrogen is of particular interest, the present invention is not limited to this aspect but is applicable wherever a first component (which may be an oxidizable or an oxidizing compound) is capable of forming a xe2x80x9ccombustiblexe2x80x9d i.e. exothermally reactive mixture with a second component which provides the complement of the reaction, i.e. is oxidizing for an oxidizable first component, orxe2x80x94in reversexe2x80x94is oxidizable by an oxidizing first component. While elemental oxygen is a typical xe2x80x9csecondxe2x80x9d component because of its universal appearance in air, other oxidizing substances, notably those having a normally (i.e. under normal conditions of temperature and pressure) gaseous phase, such as fluorine or bromine, are capable of forming exothermally reactive gaseous mixtures with such oxidizable gases as hydrogen as well. Accordingly, the term xe2x80x9ccombustionxe2x80x9d is neither to be understood as a limitation to reactions involving oxygen as the oxidizing component, nor as a limitation to monitoring of oxidizable components of mixtures.
Consequently, the term xe2x80x9ccritical first componentxe2x80x9d as used herein is intended to refer generally to that particular and potentially appearing component of an environment or atmosphere of interest which is monitored so as to indicate formation of a combustible mixture with a second component which may be but need not be present in the environment of interest.
Examples of oxidizable and typically gaseous substances of particular interest herein other than hydrogen are low molecular hydrocarbons (methane, ethane, propane, butane, singly or in mixtures) as well as other organic or inorganic substances including low boiling organic compounds, cyanogen, carbon monoxide, hydrogen sulfide etc., which can form combustible mixtures with a reactively complemental component, such as typically oxygen, capable of fast or xe2x80x9cexplosivexe2x80x9d reaction.
The concentrations, or concentration limits, at or within which a given pair of an oxidizable and an oxidizing component forms an explosive mixture are known in the art of chemical processing and do not require specific explanation herein.
Various apparatus and method means for determining the concentration of a component of a gaseous mixture are known and operate on various principles of detection, including measurement of thermal conductivity, measurement of the heat of combustion by calorimetry using, for example, a pellistor, determination of oxidizable components of an atmosphere, and methods based on the modification of electrical conductivity of semiconductors caused by the electron donating properties of oxidizable substances, cf. EP-A-0 429 397 or EP-A-0 607 756 and the literature cited therein; such prior art means could, in theory, be used for monitoring of combustible atmospheres.
However, such prior art methods have not proved to be entirely satisfactory for monitoring potentially combustible atmospheres; disadvantages include inherent difficulties of avoiding the risk of igniting the atmosphere of interest, sensitivity to interference by oxygen or other impurities, notably when electrochemical methods or the use of semiconductor are concerned, as well as insufficient stability of the monitored parameters, or lack of precision and/reproducibility and, hence, poor reliability.
Prior art amperometric means for measuring elemental gaseous hydrogen are disclosed, e.g. in U.S. Pat. No. 4,563,249. Continuous monitoring of potentially hazardous hydrogen/oxygen mixtures as disclosed, for example, in U.S. Pat. Nos. 4,906,339 and 4,985,130 operates on the principle of selectively measuring the concentration of either component in the presence of the other and could indicate the formation of an explosive mixture. Electrolytes are required in these methods, however, and if the electrolyte is aqueous, prolonged operation in a dry gaseous environment causes evaporation of water and requires frequent replacement of the electrolyte and precludes unattended operation for prolonged periods of time; non-aqueous electrolytes, on the other hand, are not satisfactory because they cause a rapid drift of the monitoring signal.
Non-amperometric monitoring methods and apparatus such as disclosed in U.S. Pat. Nos. 5,144,831 and 5,255,553 (also termed pulse monitoring herein) while capable of monitoring various substances that were difficult to monitor by prior art methods are operable with binaryxe2x80x94or at least pseudo-binary-mixtures and become less reliable when the substance to be monitored is accompanied by interfering components.
Accordingly, a first general object of the present invention are apparatus and method means for monitoring potentially explosive environments (including atmospheres within an essentially enclosed space) while avoiding or minimizing the above mentioned disadvantages of prior art, yet providing economic and completely safe apparatus and method means for continuously analyzing or monitoring mixtures that are potentially
WO-A-9 602 826 discloses a device and method for measuring the amount of a reactive gaseous component in a gas or gas mixture contained within a voluminous enclosed space containing a catalyst capable of exothermally reacting with the gas of interest and measuring the heat generated by the catalytically induced reaction of the reactive gaseous component. The rate of combustion is that of a catalytically induced reaction and does not approach explosive combustion initiated by ignition of an explosive gas mixture. In fact, such explosive combustion would neither be practical in a voluminous chamber nor yield any significant data.
GB-A-1 427 515 discloses a device and a method based on a similar concept of a catalytically induced reaction caused by a catalyst arranged within a voluminous space and containing an embedded thermometer element for measuring any temperature caused by reaction of the gas of interest with catalyst. Again, explosive combustion would not be practical nor yield any significant data.
Accordingly, a first object of the invention is an improved method of monitoring potentially explosive environments (including atmospheres within an essentially enclosed space) avoiding the disadvantages of prior art and provide for a reliable and safe test for explosive combustibility of a gas mixture by actually attempting rapid, i.e. explosive combustion in a continuous sequence of test and measuring cycles, each cycle including an actual ignition, and measuring the composition of the gas sample before and after ignition because any combustion of this type will have a most distinct impact upon the composition of the gas mixture contained in the measuring chamber so as to yield reliable and significant data, providing apparatus and method means for continuously analyzing or monitoring gaseous mixtures that are potentially or actually hazardous because of their propensity for very rapid and, sometimes, virtually spontaneous combustion, and which suffer no interference from humidity, nor the presence of oxygen or other impurities typical for a specific field of application.
A further object of the invention are apparatus and method means for analysis of gas mixtures by thermal conductivity monitoring, said mixtures containing a combustible component in the presence of at least one other substance which might interfere with the chosen method of analysis, and in the presence of non-interfering other substances, wherein measurement is effected according to the pulsing method specified in more detail below.
These and further objects and advantages as will become apparent from the following specification are achieved, according to a first embodiment of the present invention, by an apparatus for determining the concentration of a critical first component, such as hydrogen, of an environment, when the first component is capable of forming a combustible mixture with a second component, such as oxygen; said apparatus comprising:
(A) a sampling unit including:
(A1) a small measuring chamber operatively connected with an ignition means capable of being operated so as to initiate in said chamber a combustion of the critical first component by reaction with the second component, and
(A2) at least one sensor operatively connected with the measuring chamber for generating a signal formed by essentially proportionate contributions from all significant (i.e. contributing to the measurable parameters of interest) components contained in the measuring chamber;
(B) at least one flame arrestor positioned between the measuring chamber and the environment or atmosphere of interest; and
(C) a control unit capable or imposing a mode of operation in at least two distinct phases wherein, during a first phase, the environment or atmosphere of interest is allowed to accumulate in the measuring chamber; and wherein, during a second phase, the ignition means is operated for initiating combustion in the chamber; and
(D) a means for evaluating at least one signal generated by the at least one sensor before operation of the ignition means, and at least one signal generated by the at least one sensor after operation of the ignition means so as to generate a signal which is indicative of the concentration of the critical first component in the environment or atmosphere of interest.
The apparatus may include means (E) for cyclically flushing the chamber with a gas, such as air, which is free of the critical first component but may contain the second component, such as oxygen. In other words, the second component required for explosive combustion of the first component may bexe2x80x94but need not bexe2x80x94contained in the environment of interest.
According to a second preferred embodiment the invention provides for a method of determining the concentration of a critical first component of an environment or atmosphere of interest, said first component being capable of forming a combustible mixture with a second component; the method according to the invention comprises:
(a) providing a small receiving volume in operative connection with an ignition means capable of initiating combustion, in the receiving volume, of a mixture containing the critical first component and the second component;
(b) providing means between the receiving volume and the environment or atmosphere of interest so as to prevent that combustion within the receiving volume will initiate combustion outside the receiving volume and in the environment or atmosphere of interest;
(c) permitting, during a sampling phase, a portion of the environment or atmosphere of interest to enter into the receiving volume;
(d) providing at least one sensor in operative connection with the receiving volume so as to generate signals formed by essentially proportionate contributions from all significant components contained in the receiving volume;
(e) operating the ignition means so as to initiate combustion of the mixture of the first and the second component in the receiving volume;
(f) obtaining from the at least one sensor at least one signal before, and at least one signal after, operating the ignition means; and
(g) processing the signals obtained before and after operation of the ignition means to generate a signal which is indicative off the concentration of the critical first component in the environment or atmosphere of interest.
The method according to the invention may include the additional step of flushing the receiving volume with a gas, such as air, which is substantially free of the critical first component, such as hydrogen.
The term xe2x80x9cessentially proportionate contributionsxe2x80x9d with reference to the response of the at least one sensor is intended to indicate that the contribution of each component of the mixture in the measuring chamber should be proportionate to the relative amounts of the components. Preferably, the contributions are additive, and it is even more preferred if such contributions are linearly additive. The preferred sensors disclosed herein satisfy this requirement. Generally, monitoring according to the invention is effected in a prolonged if not xe2x80x9cendlessxe2x80x9d sequence of cycles, each of which includes at least two-phases, i.e. before and after each operation of the ignition means, and such cycles will be repeated as long as the monitoring process is to be continued. Cycling times can be chosen as appropriate for a given purpose; typical cycling times may be in the range of from 1 second or less to 1 minute or more, neither limit being considered to be critical except that the response or sensitivity of the monitoring operation may suffer if cycling times are excessive. Each cycle may, however, include one or more additional phases as will be explained in more detail below.
The term xe2x80x9cin operative connectionxe2x80x9d is intended to refer either to direct connection, e.g. the ignition means or sensor is arranged within the measuring chamber, or receiving volume, or is connected therewith, e.g. by a conduit. In a similar manner, the measuring chamber may be positioned within the atmosphere of interest, or be connected therewith by way of one or more conduits.
The term xe2x80x9csmallxe2x80x9d with reference to the measuring chamber of the apparatus according to the invention, or with reference to the volume of the receiving space used in the inventive method, is intended to refer to a volume of the chamber or receiving space that is substantially negligible in relation to the volume of the environment or atmosphere of interest outside of, or connected with, the measuring chamber or receiving space. In absolute terms, a typical measuring chamber or receiving space according to the invention has a volume of less than 1 ml, typically about 0.1 ml. so as to minimize the quantity of energy released by combustion of the mixture of reactive components in the measuring chamber or receiving space.
Typically, the volume of the environment or atmosphere of interest is substantially greater, say, by more than two magnitudes than the volume of the chamber or receiving space, or, in reverse, the measuring chamber or receiving volume typically will be smaller than 1%, preferably lower than 0.1% of the volume of the atmosphere of interest.
The measuring chamber, or the walling which encompasses the receiving volume, preferably is made of a material that under operating conditions is essentially inert, both physically and chemically, andxe2x80x94preferablyxe2x80x94has a high thermal capacity such as typically stainless steel. Accordingly, stainless steel is a preferred but not limiting example of a material forming the measuring chamber or surrounding the receiving volume.
Generally, the parameters of the measuring chamber or receiving volume should be selected such that a temperature rise caused by combustion of the mixture of reactive components contained in the measuring chamber is reduced to negligible proportions. This enhances the safety of the apparatus because it may help to avoid undesirable heating of surfaces in contact with the environment or atmosphere of interest. Further, the interpretation of a change of signal caused by combustion is simpler if there is no significant temperature rise to be accounted for.
Combustion of a mixture of reactive components within the measuring chamber, or receiving volume, must not initiate combustion in the atmosphere of interest. For this purpose, at least one intervening means (termed xe2x80x9cflame arrestorsxe2x80x9d herein) is positioned between the measuring chamber, or receiving volume, and the environment or atmosphere of interest. Suitable flame arrestors in the case of gaseous components may be formed by a diffusion barrier on the type known in the art, e.g. a sintered body such as a sintered plate, preferably consisting of a sintered inorganic material, e.g. a metal, such as sintered steel, or a sintered ceramic.
Alternatively, or complementally, the at least one flame arrestor may be formed by a conventional closure means, such as a valve, considered to be a xe2x80x9cstaticxe2x80x9d flame arrestor. Further, a membrane of the type specified in the above mentioned U.S. Pat. Nos. 4,906,333, 4,985,130 and 4,563,249 may be provided between the environment or atmosphere of interest and the receiving space. Such membranes are termed xe2x80x9csemi-permeablexe2x80x9d because they are capable of separating a normally liquid substance from a normally gaseous substance and are required if measurement is effected according to the pulsing method disclosed in U.S. Pat. Nos. 5,144,831 and 5,255,553, the disclosure of both of which is incorporated herein by way of reference. The membrane can be incorporated into the flame arrestor or be arranged separate therefrom in a manner preventing damage by combustion within the measuring chamber or receiving space.
Depending upon the choice of a xe2x80x9cdynamicxe2x80x9d flame arrestor in the sense of permitting continuous passage of gas from the atmosphere of interest into the measuring chamber, or receiving volume, as is the case when using a diffusion barrier, or a xe2x80x9cstaticxe2x80x9d flame arrestor, such as a valve, which permits interruption of the passage of the components of the environment of interest into the measuring chamber or receiving space, sampling will be continuous or discontinuous. With discontinuous sampling, control of the static flame arrestor can be effected advantageously by the control unit which may also control ignition time and sensor operation. Suitable control units are available commercially or may be built from commercially available elements.
A preferred type of ignition means is a heated wire or an electrical discharge device of the type forming an are in the manner of spark plugs used for internal combustion engines. Such ignition means as well as their operation is well known in the art of combustion engines and needs no specific explanation.
The at least one sensor can be a conventional device for measuring the specific parameter, generally a parameter that is capable of indicating a composition, and can be selected in view of commercial availability and specific operative requirements. Commercially available sensors capable of measuring a thermal conductivity, or a pressure, of a combustible mixture in the measuring chamber, are mentioned by way of example but are not limiting. Obviously, relatively small sensors will be preferred in view of the small volume of the measuring chamber or of the receiving volume. More than one sensor could be used, in or connected with, the measuring chamber, and the same or different parameters could be measured by two or more sensors. By the same token, operation cycles could be the same or different if more than one sensor is used. Use of a thermal conductivity sensor is preferred for many applications of the invention, notably when measurement is made according to the pulsing method mentioned above.
Both apparatus and method according to the invention are suitable either for measuring the concentration or an oxidizable and preferably gaseous component, such as hydrogen or low molecular hydrocarbons, as the critical first gaseous component in an environment or atmosphere containing or receiving an oxidizing and preferably gaseous component, such as elemental oxygen or air, as the second reactive component; alternatively, both method and apparatus according to the invention could be used for measuring the concentration of an oxidizing component as the critical first component in an environment or atmosphere containing or receiving an oxidizable reactive component, such as hydrogen or a low molecular hydrocarbon, as the second component if monitoring of the oxidizing component by conventional methods such as amperometry, should present problems.
Generally, the invention is based on an interpretation of the chance of a signal generated by the at least one sensor caused when ignition causes combustion within the measuring chamber, or receiving space. This will be explained in more detail as applied to the specific case of an atmosphere containing elemental hydrogen (H2) but similar considerations would apply to other environments or atmospheres. Again, sensors based on the measurement of thermal conductivity art of pressure are used for exemplification but other detection principles would be equally applicable in conjunction with the invention.